1. Field of the Invention
Embodiments of the invention relate to a liquid crystal display (LCD) device, and more particularly, to a method of aligning an alignment layer for an LCD device being capable of controlling a pre-tilt angle of a liquid crystal layer and an LCD device having an alignment layer aligned by the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are the subject of significant research and development because of their low power consumption and high value. Among the known types of LCD devices, active matrix LCD (AM-LCD) devices, which have thin film transistors (TFTs) arranged in a matrix array, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images. Each of the TFTs can be controlled to have ON or OFF state.
Generally, the LCD device is fabricated though an array substrate fabricating process, a color filter substrate fabricating process and a liquid crystal (LC) cell process. In the array substrate fabricating process, array elements, such as a TFT, a pixel electrode and so on, are formed on a first substrate. In the color filter substrate fabricating process, a color filter and a common electrode are formed on a second substrate. In the LC cell process, after the first and second substrates are attached, an LC layer is provided between the first and second substrates.
FIG. 1 is an exploded perspective view of a related art LCD device. The related art LCD device includes first substrate 12, second substrate 22, and a liquid crystal layer 30. The first and second substrates 12 and 22 face each other, and the liquid crystal layer 30 is interposed therebetween.
The first substrate 12 includes a gate line 14, a data line 16, a TFT “Tr”, and a pixel electrode 18. The gate line 14 and the data line 16 cross each other such that a pixel region “P” is defined between the gate and data lines 14 and 16. The TFT “Tr” is formed adjacent to a crossing of the gate and data lines 14 and 16, and the pixel electrode 18 is formed in the pixel region “P” and connected to the TFT “Tr”.
The second substrate 22 includes a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 has a lattice shape to cover a non-display region of the first substrate 12, such as the gate line 14, the data line 16, the TFT “Tr”. The color filter layer 26 includes first sub-color filters 26a, second sub-color filters 26b, and third sub-color filter 26c. Each of the sub-color filters 26a, 26b, and 26c has one of red, green, and blue colors R, G, and B and corresponds to the each pixel region “P”. The common electrode 28 is formed on the black matrix 25 and the color filter layers 26 and over an entire surface of the second substrate 22. The first substrate 12, which includes the TFT “Tr”, the pixel electrode 18 and so on, is referred to as an array substrate 10, and the second substrate 22, which includes the color filter layer 26, the common electrode 28 and so on, is referred to as a color filter substrate 20.
The LCD device can be fabricated through following processes.
First, an array pattern including a plurality of switching elements, a plurality of pixel electrodes, a gate line, a data line and pads is formed on the first substrate. The array pattern can be formed through a deposition process, a photo-lithography process and an etching process. The gate and data lines cross each other to define a pixel region. Each switching element is disposed each pixel region. Each of the plurality of pixel electrodes corresponds to each pixel region and is connected to the switching element. The pads are disposed at an end portion of the gate line and the data line. This process can be referred to as an array substrate fabricating process.
Meanwhile, a black matrix and a color filter layer including red, green and blue color filter patterns are formed on the second substrate. A common electrode is formed on the black matrix and the color filter layer to face the pixel electrode on the first substrate. This process can be referred to as a color filter substrate fabricating process.
Next, a liquid crystal layer is interposed between the first and second substrate, and then the first and second substrate are attached to each other such that a liquid crystal panel is fabricated. This process can be referred to as a cell process.
The LCD device uses electro-optical properties of liquid crystal molecules. The electro-optical properties result from optical anisotropy and arrangement of the liquid crystal molecules. Accordingly, high quality images are obtained by control of the arrangement of the liquid crystal molecules. To control an initial arrangement of the liquid crystal molecules, an aligning process is performed on an alignment layer.
The alignment process includes an alignment layer forming process and an aligning process on a surface of the alignment layer. In the alignment layer forming process, an alignment material is coated onto a substrate to form the alignment layer. In the aligning process, the alignment layer is treated to form a polymer chain arranged along one direction.
Particularly, in the alignment layer forming process, an alignment material, for example, polyimide, is coated onto a substrate to form the alignment layer having uniform thickness. The substrate can be one of the array substrate and the color filter substrate. In more detail, the alignment layer is disposed at an active region where the liquid crystal layer is formed. Accordingly, when the alignment layer is formed over an entire surface of the substrate by a spin coating method, an etching process is required to remove a portion of the alignment layer at a non-display region about the periphery of the active region. Accordingly, to avoid any additional processes, such as the etching process, the alignment layer is formed on the active region but not on the non-display region by using a transcription plate. The transcription plate is already patterned to correspond to the active regions on the substrate.
Next, the substrate including the alignment layer is treated in a drier and a hardening apparatus to remove moisture in the alignment layer and to achieve a desired hardness.
Next, a surface of the alignment layer is treated to form a polymer chain arranged along one direction on the surface of the alignment layer. The treatment process can be referred to as a rubbing process.
Hereinafter, a related art rubbing process is explained with reference to the accompanying drawings.
FIGS. 2A and 2B are plane views and a cross-sectional view showing a related art rubbing process, respectively. In FIG. 2A, a substrate 40 where an alignment layer (not shown) is disposed on a stage of a rubbing apparatus. And then, a rubbing roll 50 is disposed over the substrate 40 and rotated. Rubbing cloth, which is formed of rayon, is wound on the rubbing roll 50. Referring to FIG. 2B, when the rubbing roll 50 is rotated, the rubbing cloth 55 contacts and rubs a surface of the alignment layer 45. When the rubbing cloth 55 contacts the alignment layer 45, the stage 30 or the rubbing roll 50 moves along a direction such that an entire surface of the alignment layer 45 is rubbed by the rubbing cloth 55. As a result, a polymer chain, which is referred to as a side chain, is arranged along a direction on a surface of the alignment layer 45. Due to the side chain, liquid crystal molecules on the surface of the alignment layer 45 have a pre-tilt angle with respect to the alignment layer 45. The pre-tilt angle is defined as an angle between a major axis of the liquid crystal molecule and a surface of the alignment layer or the substrate.
In a twisted-nematic (TN) mode LCD device and an in-plane switching (IPS) mode LCD device, the pre-tilt angle can be 0 to 3 degrees. Generally, a horizontal type alignment layer is rubbed such that the liquid crystal molecule has a pre-tilt angle of 0 to 3 degrees. Before the alignment layer is rubbed, a side chain of the horizontal type alignment layer is substantially horizontal to the surface of the alignment layer. On the other hand, in a vertical type alignment (VA) mode LCD device, the liquid crystal molecule has a pre-tilt angle of 89 to 90 degrees. The vertical type alignment layer is first formed and then rubbed.
Recently, a new mode of LCD device has been introduced in which the alignment layer is required to be rubbed to have a pre-tilt angle of 20-70 degrees. Namely, the alignment layer is required to be capable of having a controllable pre-tilt angle. To meet the requirement, blending of polymer materials having different properties have been researched. However, since the blending is very difficult, there is no practical use. Moreover, a controllable pre-tilt angle can be obtained by forming a horizontal type alignment layer and a vertical type alignment layer with controlled rubbing densities on each of them. However, such a process requires forming alignment layers at least twice. Accordingly, there are problems that production time and production costs increase.
In addition, a controllable pre-tilt angle may be obtained by using transcription plates having different patterns. However, the transcription plate has ductility such that there is no reliability.
On the other hand, at least two rubbing processes cause some other problems. When the rubbing cloth contacts the alignment layer, a hair of a surface of the rubbing cloth is separated such that particles are generated. Moreover, because the rubbing cloth itself generates fine dusts, there are some problems in a fabricating process of the LCD device where an excellent cleaning is required. Further, disconnection of the electrical lines on the substrate and degradation of properties in the switching element can occur due to static electricity being generated between the rubbing cloth and the alignment layer. Furthermore, as the substrate becomes larger, the rubbing cloth is required to be longer. When the longer rubbing cloth is rotated, an eccentric force increases that causes vibrations in the rubbing roll to also increase. Such vibrations cause the aligning properties of the resultant alignment film to be non-uniform.
Recently, the LCD device is required to have fast response time and a wide viewing angle. To meet these requirements, an optically compensated bend (OCB) mode LCD device has been introduced. Due to symmetrical arrangement in liquid crystal molecules with respect to a center line in a liquid crystal layer, a compensating plate is not required. Accordingly, there is an advantage in production cost.
FIGS. 3A to 3C are schematic cross-sectional views showing arrangements of liquid crystal molecules in a related art OCB mode LCD device, respectively. FIGS. 3A to 3C respectively show a splay state, a bend-I state and a bend-II state.
The OCB mode LCD device includes a first substrate 60, a second substrate 70 and a liquid crystal (LC) layer 80 therebetween. A first alignment layer 62 is disposed over the first substrate 60, and a thin film transistor (not shown) as a switching element and a pixel electrode (not shown) are disposed between the first alignment layer 62 and the first substrate 60. A second alignment layer 72 is disposed over the second substrate 70, and a color filter layer (not shown) and a common electrode (not shown) are disposed between the second alignment layer 72 and the second substrate 70. The LC layer 80 including liquid crystal (LC) molecules 82 and 84 are disposed between the first and second alignment layers 62 and 72.
In FIG. 3A showing the splay state where voltages are not applied, first LC molecules 82 adjacent to one of the first alignment layer 62 and the second alignment layer 72 are symmetric to each other with respect to a second LC molecule 84 in a center of the LC layer 80. In this case, a first pre-tilt angle θ1 of the first LC molecules 82 is about 1 degree to about 3 degrees with respect to one of the first and second alignment layers 62 and 72 (or first and second substrates 60 and 70). On the other hand, the second LC molecule 84 is substantially parallel to one of the first and second alignment layers 62 and 72 (or first and second substrates 60 and 70).
When a first voltage is applied, the OCB mode LCD device has the bend-I state in FIG. 3B. The first voltage may be referred to as an initial voltage. In the bend-I state, the OCB mode LCD device has an ON state and displays a white image. In this case, the first LC molecules 82 has a second pre-tilt angle θ2 greater than the first pre-tilt angle θ1 in the splay state. The second LC molecule 84 is arranged to be substantially perpendicular to one of the first and second substrates 60 and 70.
On the other hand, when a second voltage being greater than the initial voltage is applied, the OCB mode LCD device has the bend-II state in FIG. 3C. The second voltage can be referred to as a driving voltage. In the bend-II state, the OCB mode LCD device has an OFF state and displays a black image. In this case, the first LC molecules 82 has a third pre-tilt angle θ3 greater than the second pre-tilt angle θ2 in the bend-I state. The second LC molecule 84 is arranged to be substantially perpendicular to one of the first and second substrates 60 and 70.
Referring to FIG. 4 showing relation between states of the OCB mode LCD device and applying voltages, since the OCB mode LCD device can displays images in the bend-I state and the bend-II state, it has fast response time. Namely, when the driving voltage is applied to the LC molecules in the bend-I state where the LC molecules adjacent to the first and second alignment layers have the second pre-tilt angle being greater than the first pre-tilt angle in the splay state, it is possible to obtain fast response time. However, since the LC molecules have to be the bend-I state, it is required to apply the initial voltage, which causes an increase in power consumption.